Thermo-electrical performance and heat transfer mechanisms of an immersion battery thermal management system based on synthetic ester fluid

Efficient thermal management is essential to ensure the safety, durability, and performance consistency of lithium-ion batteries in electric vehicles. This study develops an immersion battery thermal management system (IBTMS) based on synthetic ester fluid and systematically investigates its thermo-electrical behavior and underlying heat transfer mechanisms. An integrated experimental platform incorporating a 1P8S cylindrical battery module is established to evaluate the coupled thermal and electrical characteristics under various discharge rates (0.5C–3C) and thermal fluid flow rates (0–1000 mL/min). Experimental results demonstrate that, compared with air battery thermal management system, the IBTMS based on synthetic ester fluid reduces the ΔTm-max, ΔTb-max and θmax by 74.52%, 60.83% and 79.93% at a 3C discharge rate. The Pearson correlation coefficient (PCC) analysis reveals a strong positive relationship (r = 0.95) between temperature nonuniformity and voltage deviation, highlighting the dominant role of thermal gradients in electrical imbalance. A comprehensive heat transfer model considering natural, forced, and mixed convection is developed, and an empirical correlation NuFC = 15.130Re0.694 is derived with an average deviation of 2.21%. Under the World Light-duty Vehicle Test Cycle (WLTC) conditions, the proposed system maintains stable operation, achieving an 11.2% reduction in peak temperature and improved voltage uniformity. These findings provide new experimental evidence and modeling insights for the integrated design of the immersion battery systems based on synthetic ester fluid.